There has been considerable interest in recent years in the use of biodegradable polymers to address concerns over plastic waste accumulation. The potential worldwide market for biodegradable polymers is enormous (&gt;10 B lbs/yr.). Some of the markets and applications most amenable to the use of such biopolymers involve those having single, short use applications, including packaging, personal hygiene, garbage bags, and others. These applications, although poorly suited for recycling, are ideally suited for biodegradation through composting.
PHA biopolymers are thermoplastic polyesters produced by numerous microorganisms in response to nutrient limitation. The commercial potential for PHA spans many industries, and is derived primarily from certain advantageous properties which distinguish PHA polymers from petrochemical-derived polymers, namely excellent biodegradability and natural renewability.
Widespread use and acceptance of PHA, however, has been somewhat hindered by certain undesirable chemical and physical properties of these polymers. For example, PHA is perhaps the most thermosensitive of all commercially available polymers. As such, a dramatic rate of polymer degradation is observed at the temperatures typically required for conventional processing of PHA into end-products such as films, coating, fibers etc. An additional limitation of the potential utility of PHA polymers relates to the observation that some characteristics of the polymer (for example, ductility and permeability) diminish over time. This rapid "aging" of certain PHA-derived products is unacceptable for many applications. Thus, the success of PHA as a viable alternative to both petrochemical-derived polymers and to non-PHA biodegradable polymers, will depend upon novel approaches to overcome the unique difficulties associated with PHA polymers and with products derived therefrom.
One important application of biodegradable polymers is in the area of paper coating for packaging and one time use applications. In many applications, coated papers need to be heat sealed. The adhesive strength of the heat sealed material and its stability under different conditions is critical. However, prior to the present disclosure, the production of PHA coating materials having acceptable heat sealability had not been addressed.
WO 94/16000 discloses a process for preparing a film (or coating) of PHA comprising applying a layer of molten PHA to a surface not substantially above the Tg of the PHA to form a film and subsequently raising the temperature to Tc, the optimum temperature for crystallization, to produce a non-sticky film (or coating). In one example, a PHBV (10% valerate) containing 1 phr boron nitride (BN) at 180.degree. C. was extrusion coated onto paper at a line speed of 60 m/min with the chill roll at 4.degree. C., then passed through an IR-heated zone at 80.degree. C. to crystallize the PHA as a 10 micron thick coating, and then wound onto a roll.
WO 95/15260 discloses a process for making biodegradable film containing at least one layer comprising cellulose and at least one layer comprising PHA, with heating to cause adherence between layers. The PHA is applied as a solution or aqueous suspension or latex. The reference also discloses a process of laying PHA sides of two films together and heat sealing them.
WO 95/17454 discloses a process for preparing extruded PHA films and coatings by applying a molten layer of the composition to a surface which is at a temperature in the range from 20.degree. C. above to 20.degree. C. below the optimum crystallization temperature of the polymer. The process is disclosed as being suitable for producing PHA films at higher line speeds than previously possible in a single stage process.
Lacking from the prior art, however, are methods of producing PHA coatings having sufficient heat sealing properties as required for forming shaped articles such as cups and other containers where heat sealability is important. The present invention has identified unexpected molecular weight requirements of PHA in coating compositions that are required in order to achieve acceptable heat sealing, and the means by which to arrive at the coatings.